Method for burner and burner device

ABSTRACT

Burner device ( 2 ) for an industrial furnace ( 1 ) comprising a channel ( 3 ) for fuel, a channel ( 4 ) for a first oxidant, an outlet ( 9 ) for flue gases, a control device ( 10 ) and a heat buffer ( 7 ), where the first oxidant and the flue gases alternatingly are led through the heat buffer ( 7 ). The invention is characterised in that a separate lancing device ( 6 ) is arranged to supply a second oxidant to the burner device ( 2 ).

The present invention relates to a method and a burner device forincreasing the efficency during heating of an industrial furnace. Morespecifically, the invention relates to a so called regenerative burnerwith improved combustion efficiency.

Herein, the expression “regenerative” burner device is used for a burnerdevice in which the cold oxidant and hot flue gases alternatingly areled through a heat buffer, so that the heat of the flue gases is usedfor preheating of the oxidant. This increases efficiency duringcombustion.

For example, the heat buffer may consist of a pipe partially filled withheat absorbing material, such as balls or a honeycomb structure madefrom a ceramic or a metal material. A commonly used operation schemeinvolves about 30 seconds of percolation of hot flue gases through thebuffer, followed by about 30 seconds of percolation of ambientlytempered oxidant, which oxidant is thereby heated by the heat absorbingmaterial. The combustion may be alternated between two or more burners,whereby hot flue gases from the other burners are led out from thefurnace through the oxidant channel of one of the burners at a time andthrough the heat buffer, or one burner may be furnished with severaloxidant channels that are used alternatingly to supply oxidant to theburner and carry flue gases out from the furnace.

Such regenerative burner devices are typically optimised for a specific,high power. This works well for continuous heating, but is problematicfor batchwise heating. Namely, batch-wise heating of metal materialoften takes place over two steps, where an introductory, high powerstep, herein denoted “heating step”, aims at heating the furnace and thesurface of the metal material to a predetermined final temperature, anda final, low power step, herein denoted “temperature homogenising step”,aiming at letting the whole metal material assume a predetermined,homogenous temperature profile through heat conduit.

The heating step may for instance last between 2 and 10 hours, dependingon the prerequisites, during which time the burner or burners is or areoperated at or near their optimal, high power. The temperaturehomogenising step may correspondingly last between 1 and 40 hours, andinvolves considerably lower burner powers.

During parts of the heating step, the furnace temperature is essentiallylower than the desired process temperature, why the heat absorbingmaterial in the heat buffer cannot be heated to the temperature which isneeded for the burner to reach maximum efficiency.

Furthermore, regenerative burners being dimensioned for the heating stepare at the same time overdimensioned for the temperature homogenisationstep, why their power must be adjusted to be lower during this step. Byway of example, this may be done by using a so called on/off procedure,in which the burner alternatingly and periodically is turned on or off.Among other things, this leads to that the temperature in the heatbuffer decreases during the switched off time, which substantiallyreduces the efficiency of the burner.

Because of the above described considerations regarding choice ofoptimal combustion power, regenerative burners are often designed with amaximum power which is near that needed during the heating step. As aconsequence, substantial costs and/or construction changes are needed ifan even higher power for some reason is needed at a later time, sinceregenerative burners are comparatively large and bulky by design, andsince each such burner also normally demands correctly dimensioned fandevices for conveying the gases through the heat buffer, which fanstherefore also need to be changed.

The present invention solves the above described problems.

Thus, the invention relates to a burner device for an industrial furnacecomprising a channel for fuel, a channel for a first oxidant, an outletfor flue gases, a control device and a heat buffer, where the firstoxidant and the flue gases alternatingly are led through the heatbuffer, and is characterised in that a separate lancing device isarranged to provide a second oxidant to the burner device.

Furthermore, the invention relates to a method of the type and withmainly the features as described in claim 11.

In the following, the invention will be described in detail, withreferance to exemplifying embodiments of the invention and to theappended drawing, where:

FIG. 1 is an overview from the side principally showing a burner deviceaccording to the invention.

In an industrial furnace 1 for batchwise heating of metal material aregenerative burner device 2 is arranged. More than one burner devicemay be arranged inside the furnace 1, depending on the actualapplication. Especially, burner devices may be arranged in pairs in theindustrial furnace 1, whereby such burner pairs may be operatedalternatingly.

The burner device 2 comprises a channel 3 for fuel and a channel 4 for afirst oxidant. The fuel may be any suitable, gaseous fuel, such aspropane or natural gas. The first oxidant is for example air, but mayalso be oxygen enriched air or oxygen gas with an oxygen content of atleast 80 percentages by weight. The burner device 2 gives rise to aflame (not shown), which is directed into and therefore heats thefurnace space. Moreover, the burner device 2 may be of the DFI (DirectFlame Impingement) type, and/or of so called flameless type, whereby avisual flame is missing.

Furthermore, the burner device 2 comprises a supply conduit 5 for fueland a supply conduit 8 for the first oxidant. The first oxidant issupplied through the supply conduit 8 and into a heat buffer 7,containing a heat absorbing material such as described above, andthereafter on to the channel 4.

An outled conduit 9 for flue gases also runs from the heat buffer 7.

The supply conduit 5 for fuels the supply conduit 8 for the firstoxidant and an outlet conduit 9 for flue gases are all connected to acontrol device 10 that controls the operation of the burner device 2.

The control device 10 may control the burner device 2 for operationeither in combustion mode or in reheating mode. In combustion mode, fueland the first oxidant are supplied through the conduits 5 and 8,respectively, and the mixture is combusted so that the furnace space isheated. In reheating mode, neither fuel nor the first oxidant aresupplied, but flue gases are conveyed out from the furnace space,through the channel 4 for the first oxidant, on to the heat buffer 7 andout through the outlet conduit 9, for subsequent removal. Thus, the heatabsorbing material in the heat buffer 7 is heated by the hot flue gasesduring reaheting mode operation, and the room tempered, first oxidant ispreheated by the same material in the heat buffer 7 during combustionmode operation.

The control device 10 controls the inbound and outbound gas streams in asuitable manner, for instance using a valve arrangement (not shown)and/or suitable, known fans (not shown).

The burner device 2 may also comprise a separately arranged outletconduit for flue gases that run out from the furnace space and into theheat buffer 7.

During operation, the control device 10 accomplishes that the burnerdevice 2 is alternated between combustion mode and reheating mode,whereby the thermal energy in the flue gases is recovered for heating ofthe first oxidant, which increases the combustion efficiency. Either onesingle burner device may be used, which in this case only heats thefurnace space when operated in combustion mode, or several burnerdevices may be used in combination. In the latter case, one or severalburner devices may be operated in heating mode and the rest of theburner devices be operated in reheating mode, leading to that thefurnace space is heated at all times without interruption. According toa preferred embodiment, all burner devices except one in a burneraggregate are operated in combustion mode, and the last such burnerdevice is operated in reheating mode.

According to another preferred embodiment, burner devices are operatedin pairs, so that the first device is operated in heating mode at thesame time as the second is operated in reheating mode, after which theroles are swapped, and so on.

Several burners may share the same heat buffer 7. In this case, asuitable known valve arrangement is used for conveying the first oxidantthrough a first channel in the heat buffer 7 to burner devices beingoperated in heating mode, and at the same time conveying the flue gasesfrom burner devices being operated in reheating mode through a second,separate channel in the heat buffer 7 and out through the outlet conduit9. In this case, the first and the second channels are thermallyconnected, for example by sharing the same heat absorbing material inthe form of a common wall or the like.

In the channel for the first oxidant 4 there is provided a lance 6,which is fed with a second oxidant. Preferably, this second oxidant iscomprised of at least 80 percentages by weight oxygen, more preferablyat least 95 percentages by weight oxygen. Such an elevated concentrationof oxygen in the second oxidant leads to, among other things, highcombustion efficiency.

The lance 6 is also connected to, and controlled by, the control device10.

In a regenerative burner device an important factor, limiting themaximum combustion power, is the amount of preheated oxidant that may beled out through the channel for oxidant 4 per time unit. This dependsupon the capacity of the heat buffer 7 as well as on the fact that theheated oxidant has a lower density than cooler oxidant, which results ina larger volume oxidant needed in order to reach the same weight amount.To the contrary, it is comparatively easy to increase the amount ofsupplied fuel.

Since the second oxidant is supplied through the lance 6, one canincrease the maximum combustion power without having to in other waysincrease the dimensions of the components of the burner device 2,something which would lead to the above described problems withoverdimensioning during the temperature homogenising step.

Furthermore, it is preferred that the second oxidant is supplied at anelevated velocity, preferably a velocity above about 200 m/s, morepreferably a velocity which at least amounts to the acoustic velocity.This creates an ejector. effect, which reduces the pressure downstreamof the heat buffer 7, in turn resulting in an increased flow ofpreheated oxidant and thereby increased efficiency. At the same time, ofcourse the supplied second oxidant also takes part in the combustionreaction, hence resulting in a higher combustion power.

The amount of the second oxidant which is supplied through the lance 6may be controlled either by adjusting the size of the orifice of thelance 6 or by adjusting the pressure of the second oxidant beingsupplied through the lance 6, and may be chosen depending on the purposeof the actual application, in terms of for example combustion power,desired oxygen enrichment and limit amounts for various combustionproducts such as NO_(x) gases. The control may take place continuouslyduring operation, either by a continuous change of the pressure upstreamof the lance 6 or according to an on/off procedure, or a single timebefore the initialisation of a certain operational step.

Through such control, and unlike the case for known regenerative burnerdevices, it is thus possible to control the power of the burner device 2over a broad power interval, essentially without the efficiency at agiven power within this interval being deteriorated.

Therefore, the regenerative burner device 2 may be optimised foroperation also during a temperature homogenising step, when the powerdemands are relatively low, which optimisation leads to high efficiency.Thus, during this step little or no second oxidant is supplied throughthe lance 6. Since the power of the burner device 2 is low at this time,the need for using an on/off procedure for additionally decreasing thepower during this step is lessened or disappears completely.

On the other hand, during a heating step preceding the temperaturehomogenising step, when the power needs of the burner device 2 arehigher, the second oxidant is supplied through the lance 6 to acomparatively large extent. This results in the combustion powerincreasing substantially, however with preserved efficiency. As a matterof fact, the additional, second oxidant, supplied at an elevatedvelocity, results in the heating power of the burner device 2 increasingin comparison to conventional, regenerative burner devices 2, whichdecreases the time necessary for completion of the heating step.

The amount of the second oxidant that is lanced in the burner device 2may be controlled in different ways, such as is described above, but theaverage supplied amount of second oxidant per time unit is less duringthe temperature homogenising step than during the heating step.

Also, it is possible to apply the present invention to many existingburner devices with no larger alterations, except for the installationof a lance for additional oxidant in the existing burner device.Specifically, existing fan devices and other peripheral equipmentnormally need not be replaced. Hence, the invention achieves arelatively cheap way of substantially broadening the power interval overwhich high combustion efficiency may be upheld, and at the same timeachiveing an increase in the maximum power of the existing combustiondevice.

Above, preferred embodiments have been described. However, it isapparent to the skilled person that many alterations may be made to thedescribed embodiments without departing from the idea of the invention.

For example, it is possible to arrange several channels for a firstoxidant in the burner device 2. In this case, it is preferred to arrangea lance for a second oxidant in each such channel for the first oxidant.By the help of such an arrangement, a better control over the flameresulting from the burner device may be achieved, and the power of theburner device may be increased. It is also possible to arrange lancesfor the second oxidant in merely one or only in some of the channels forthe first oxidant.

Thus, the invention shall not be limited to the described embodiments,but be variable within the scope of the appended claims.

1. Burner device (2) for an industrial furnace (1) comprising a channel(3) for fuel, a channel (4) for a first oxidant, an outlet (9) for fluegases, a control device (10) and a heat buffer (7), where the firstoxidant and the flue gases alternatingly are led through the heat buffer(7), characterised in that a separate lancing device (6) is arranged tosupply a second oxidant to the burner device (2).
 2. Burner deviceaccording to claim 1, characterised in that the burner device (2) isarranged to heat metal material batchwise in the industrial furnace (1).3. Burner device according to claim 1, characterised in that the lance(6) is arranged to supply the second oxidant through the same channel(4) as the first oxidant.
 4. Burner device according to claim 1,characterised in that the first oxidant is air.
 5. Burner deviceaccording to claim 1, characterised in that the second oxidant iscomprised of at least 80 percentages by weight oxygen.
 6. Burner deviceaccording to claim 5, characterised in that the second oxidant iscomprised of at least 95 percentages by weight oxygen.
 7. Burner deviceaccording to claim 1, characterised in that the control device (10) isarranged to control the supply of the second oxidant continuously duringoperation.
 8. Burner device according to claim 1, characterised in thatthe control device (10) is arranged to control the supply of the secondoxidant using an on/off procedure.
 9. Burner device according to claim1, characterised in that the lance (6) is arranged to supply the secondoxidant at a velocity above 200 m/s.
 10. Burner device according toclaim 1, characterised in that the lance (6) is arranged to supply thesecond oxidant at a velocity which is at least the sonic velocity. 11.Burner device according to claim 2, characterised in that the lance (6)is arranged to supply the second oxidant through the same channel (4) asthe first oxidant.
 12. Burner device according to claim 3, characterisedin that the second oxidant is comprised of at least 95 percentages byweight oxygen.
 13. Burner device according to claim 2, characterised inthat the lance (6) is arranged to supply the second oxidant at avelocity which is at least the sonic velocity.
 14. Burner deviceaccording to claim 3, characterised in that the lance (6) is arranged tosupply the second oxidant at a velocity which is at least the sonicvelocity.
 15. Method for increasing efficiency when heating anindustrial furnace (1) using a burner device (2) comprising a channel(3) for fuel, a channel (4) for a first oxidant, an outlet (9) for fluegases and a heat buffer (7), in which burner device (2) the firstoxidant and the flue gases are caused to be alternatingly led throughthe heat buffer (7), characterised in that a lancing device (6) iscaused to supply a second oxidant to the burner device (2).
 16. Methodaccording to claim 15, characterised in that the heating is batchwiseheating of metal material in the industrial furnace (1).
 17. Methodaccording to claim 15, characterised in that the first oxidant is air.18. Method according to claim 15, characterised in that the secondoxidant is comprised of at least 80 percentages by weight oxygen. 19.Method according to claim 15, characterised in that, during heating ofmetal material in the industrial furnace (1), the second oxidant issupplied during a first heating step and the average supply per timeunit of the second oxidant thereafter is completely or partiallydecreased during a second temperature homogenising step.
 20. Methodaccording to claim 15, characterised in that the second oxidant issupplied at a velocity above 200 m/s.
 21. Method according to claim 15,characterised in that the second oxidant is supplied at a velocity whichis at least the sonic velocity.
 22. Method according to claim 16,characterised in that, during heating of metal material in theindustrial furnace (1), the second oxidant is supplied during a firstheating step and the average supply per time unit of the second oxidantthereafter is completely or partially decreased during a secondtemperature homogenising step.
 23. Method according to claim 21,characterised in that, during heating of metal material in theindustrial furnace (1), the second oxidant is supplied during a firstheating step and the average supply per time unit of the second oxidantthereafter is completely or partially decreased during a secondtemperature homogenising step.